In a machine tool it is desirable to ascertain and correct the relative position between a tool member and a workpiece at a given instant in the course of a machining operation to allow machining to proceed with safety and in a manner to yield desired results and machining accuracy.
For example, in an electroerosion machine tool, a tool electrode is juxtaposed with a workpiece across a machining gap flushed with a liquid machining medium and a continuous or pulsed electric current of high current density is passed through the gap between the tool electrode and the workpiece to electroerosively remove material from the workpiece. As material removal proceeds, the tool electrode and the workpiece are relatively displaced in a direction to advance the erosion process and in a manner to maintain the machining gap substantially constant in its size and desirably also the physical condition therein. It can been seen, therefore, that the precision of ascertainment and correction of the machining gap is critical to the machining results that ensue.
In the prior art, substantial efforts have accordingly been focused on seeking to provide a method of greater reliability to ascertain the gap size and condition especially in EDM processes. In one advanced method, which is disclosed in my U.S. Patent Application Ser. No. 112,460 filed Jan. 16, 1980 as a continuation of Ser. No. 860,164 filed Dec. 13, 1977 (now U.S. Pat. No. 4,236,057 issued Nov. 25, 1980), the supply of machining power pulses to the EDM gap is temporarily interrupted and, during the pwriod of interruption, a monitoring pulse is applied across the gap to measure a gap variable, e.g. impedance or resistance, from which the gap size and condition may be ascertained.
I have now determined, however, that the measurement alone of a variable of the gap condition as it actually exists at any instant during the monitoring pulse at which a command for the measurement is applied does not necessarily yield the precise indication of the gap size and condition.